A micromachined freestanding terahertz absorber with an array of metallic patches

A micromachined freestanding terahertz absorber with an array of metallic patches A micromachined freestanding terahertz absorber with an array of metallic patches Hamdi Torun, , Seyedehayda Sadeghzad[.]

Hamdi Torun, Seyedehayda Sadeghzadeh, Habib Bilgin, and Arda D Yalcinkaya

Citation: AIP Advances 6, 035323 (2016); doi: 10.1063/1.4945417

View online: http://dx.doi.org/10.1063/1.4945417

View Table of Contents: http://aip.scitation.org/toc/adv/6/3

Published by the American Institute of Physics

A micromachined freestanding terahertz absorber

with an array of metallic patches

Hamdi Torun,1,2, aSeyedehayda Sadeghzadeh,1Habib Bilgin,1

and Arda D Yalcinkaya1,2

1Department of Electrical and Electronics Engineering, Bogazici University,

Bebek 34342 Istanbul, Turkey

2Center for Life Sciences and Technologies, Bogazici University,

Kandilli 34684 Istanbul, Turkey

(Received 19 January 2016; accepted 22 March 2016; published online 30 March 2016)

An array of square metallic patches on a thin suspended dielectric layer is introduced

as an effective terahertz absorber The suspended structure is placed on a metalized substrate and the device exhibits metamaterial behavior at specific frequencies deter-mined by the size of the patches It is feasible to place patches with different sizes in

an array formation for a broadband absorber In array configuration, individual ele-ments induce distinct resonances yielding narrow band absorption regions Design of the absorber is described using electromagnetic simulations The absorber structure was fabricated on a silicon wafer using standard microfabrication techniques The characteristics of the absorber were measured using a terahertz time domain spec-troscope The measured data match well the simulations indicating strong absorption peaks in a band of 0.5-2 THz C 2016 Author(s) All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).[http://dx.doi.org/10.1063/1.4945417]

I INTRODUCTION

Metamaterials are patterned sub-wavelength sized structures that are usually made of dielec-trics and metals They exhibit strong resonant behavior when they are excited with electromagnetic waves and the resonant frequencies are determined by the geometries of the patterned structures Metamaterials with different geometries and sizes have been introduced for a wide range of bands

in electromagnetic spectrum spanning from radio1 3to terahertz,4infrared5and visible frequencies.6 The wavelength of the radiation at 1 THz corresponds to 300 µm, so it is feasible to fabricate metamaterial structures for terahertz applications using standard UV photolithography In addition, most of the materials used for the realization of metamaterials are compatible with microfabrication technology Consequently, a wide variety of metamaterial microstructures have been presented for terahertz applications Variety of imaging and spectroscopy applications in terahertz band has been increasing rapidly including detection of explosives,7surveillance,8 , 9chemical sensing,10and med-ical screening.11 , 12High performance terahertz absorbers enable developing new devices for these applications

Metamaterial-based terahertz absorbers have been employed that exhibit a magnetic or electric resonance at desired frequencies Different geometries have been demonstrated including split-ring resonators,13 , 14Swiss crosses,15 – 17concentric rings,18 square patches,19 – 23and circular disks.24 At resonance, these structures can absorb radiation with high efficiency Unity absorption is observed with metamaterial absorbers at terahertz frequencies.25 However, the absorption band is usually very narrow For example, the full width at half maximum (FWHM) of the absorption peak is 0.1 THz for an absorber at 1 THz.25Typically, structures with different sizes are combined together

to increase the number of absorption bands for broadband absorbers As an alternative method,

a Corresponding author Tel: +90 212 359 68 95 E-mail address: hamdi.torun@boun.edu.tr

FIG 1 Three-dimensional drawing of the metamaterial-based terahertz absorber structure.

square patches with comparable sizes are located in a planar array19or are stacked on top of each other with dielectric spacers in between20,23to increase the number of absorption bands In addition, concentric rings are used as resonators to extend the absorption band as well.18

In this paper, we present a metamaterial-based terahertz absorber employing metallic square patches implemented in the same plane with different sizes Fig 1 shows a three-dimensional drawing of an absorber structure on a silicon wafer The metallic patches are fabricated on top of

a thin patterned Parylene layer that is suspended over a metallized substrate Terahertz resonators

on thin dielectric layers are demonstrated for filter and sensor applications in the literature.26 , 27Our aim is to realize a terahertz absorber on a thin layer of dielectric and the intended configuration for the absorber is when the incident wave vector is perpendicular to the device as shown in Fig.1 Metallic patches with different sizes are laid out on a single Parylene film that is anchored to the substrate through a set of suspensions This configuration is desirable for terahertz detectors that requires pixel structures isolated from their substrate The metamaterial behavior is observed for unit cells including the metallic patches on top, blanket metal on the substrate and the Parylene and air spacing in between Transmission is guaranteed to be zero with the presence of the thin blanket metal underneath the patches, whereas reflection from the device diminishes at certain frequencies set by the geometry of the square patches, resulting in increased absorption at those frequencies

We implemented absorbers employing different configurations of square patch arrays made

of titanium with side lengths of 86 µm, 43 µm, 30 µm and rectangular patches with side lengths

of 100 µm and 43 µm The individual metal patches are separated by a planar gap of 2 µm The thickness of the patches and the blanket titanium film underneath the patches is 200 nm The Pary-lene layer is implemented in a 2 µm-thick released mesa The thickness of the air gap between the Parylene layer and the blanket metal is 5 µm

II SIMULATIONS AND MODELING

Fig.2(a)shows a three-dimensional drawing of a metamaterial-based terahertz absorber that includes a 4x4 array of patches with a size of 43 µm at the center of the free-standing Parylene layer The absorber also includes two rectangular patches with a size of 100x43 µm Fig.2(b)shows the reflection, transmission and absorption spectra of the absorber obtained using a commercially available electromagnetic simulation software (CST Studio Suite, Darmstadt, Germany) The prop-agation vector is normal to the surface (along z-axis, Fig.1) and the electric field vector is aligned along y-axis of Fig.1 We implemented waveguide ports perpendicular to the structure as the source

of excitation for the simulations and chose a time domain solver with hexahedral adaptive meshing

The absorption spectrum in a band of 0.3-2 THz exhibits two peaks at 0.59 THz and 1.47 THz The patch structures introduce electric dipole resonances at specific frequencies induced by the electric field in the plane of the patches At resonance, the metamaterial can be modeled using a parallel combination of an equivalent capacitance (Ceq) and inductance (Leq) given below.19

FIG 2 a) Three-dimensional drawing of a metamaterial-based terahertz absorber with a 4x4 array of patches with a size of

43 µm and two rectangular patches with a size of 100x43 µm (b) Simulated transmission, reflection and absorption spectra

of the absorber.

Ceq=αεe ffL2

teff , Leq= βµe ffte ff (1)

fr e s= 1 2π LeqCeq

(2)

where L is the side length of a square patch, te ff is the effective thickness of the structure between top and bottom metal films, α and β are geometrical correction parameters, εe ff and µe ff are the

effective permittivity and permeability of the structure, respectively

The resonant frequency of the structures is inversely proportional to the size of the patches The larger rectangular patches excites a resonance at 0.59 THz while the smaller square patches excites

FIG 3 The variation of the resonance characteristics of the structure as a function of (a) thickness of the Parylene layer, (b) gap height at the tip of the structure.

another resonance at 1.47 THz The array formation of the square patches enhances the peak of the absorption at the resonance The resonant frequency of the device is also determined by the effective relative permittivity of the structure So, the thickness of the Parylene layer has an influence on the resonance characteristics Fig.3(a)shows the distribution of the resonance peaks with respect to the changes in the thickness of the Parylene layer The effective permittivity of the structure increases with the thickness of the dielectric layer This results in a decrease in resonant frequency of the device, as expected

During the operation of the device, the freestanding structure will be tilted with respect to its substrate We analyzed the dependency of the resonator behavior by introducing a tilt angle such that the position of the edge of the structure where we place the rectangular patches is kept station-ary and the freestanding structure is tilted about the y-axis (Fig.1) We varied the gap at the tip of the structure between 3 and 7 µm and observed the resonance characteristics as shown in Fig.3(b) The resonant frequency of the structure excited by the rectangular patches keeps the same value at 0.59 THz, since the effective gap change along the rectangular patches are minimal However, the resonant frequency related to the array of the square patches decreases with increasing gap

The length of the patches along which the electric field is aligned determines the resonant frequency of the structure Thus, the resonant frequency is ideally the same for the electric field vector aligned along x-axis or y-axis of Fig.1 since the basic structure is a square We analyzed the dependency of the resonator characteristics with respect to the polarization as shown in Fig.4 The field was kept perpendicular to the device along z-axis and we varied the angle β between

FIG 4 The dependency of the resonance characteristics of the structure with respect to the orientation of electric field (a) Variation of the angle between the electric field vector and y-axis The electric field and the magnetic field vector is in the plane of xy (b) Absorbance of the structure with di fferent values of β.

the electric field vector and y-axis The absorption spectra for β= 0◦ and β= 90◦are shown in Fig 4(b) The resonant frequency of the structure at 1.47 THz due to the square patches is not altered However, the resonance at 0.59 THz due to the square patches disappears for β= 90◦since the electric field is no more aligned along the larger side of the rectangular patches On the other hand, another resonance at 1.2 THz is observed for β= 90◦as a result of the interaction between the rectangular and square patches at that frequency

Due to the fact that the resonance is mainly determined by the electric field vector along the edge of a square or a rectangular patch, the incidence angle should have minor influence on the resonance behavior as long as the orientation of the electric field vector is kept the same We analyzed the influence of the incidence angle for TE mode as shown in Fig 5(a) by varying the angle θ between the incoming beam and the z-axis The absorption spectra of the structure for various angles of θ are shown in Fig.5(b) The structure performs well as an absorber even for large incidence angles for TE mode as expected

FIG 5 The dependency of the resonance characteristics of the structure with respect to the incidence angle (a) The plane

of incidence and the angle between the propagation vector and z-axis is shown The orientation of electric field is stationary and is along y-axis (b) The dependency of the resonance characteristics of the structure with respect to the changes of the angle θ.

We designed another absorber by combining different sizes of patches on a single freestanding Parylene layer The layout of the patches is shown in Fig.1 The absorber includes square patches with side lengths of 86 µm, 43 µm, 30 µm and rectangular patches with side lengths of 100 µm and 43 µm The computational model for the absorber includes all the structures shown in Fig.1 Electric field is along y-axis for the simulations The reflection, transmission and absorption spectra

of the absorber are shown in Fig 6 The absorption spectrum exhibits sharp resonant peaks at 0.63 THz, 1.12 THz, 1.47 THz and 1.87 THz Compared to Fig.2, it can be seen that incorporating patches with different sizes improve the absorption characteristics of the device

Electric field distributions corresponding to the resonant frequencies are shown in Fig 7 Larger patches are associated with smaller frequencies Specifically patches with 100 µm side lengths excites resonance at 0.63 THz, 86 µm patch excites resonance at 1.12 THz, 43 µm patches and their interactions with 30 µm patches excite resonance at 1.47 THz The rectangular patches also excite another resonance at 1.87 THz The absorber characteristics are different when the electric field is along x-axis (see Fig.1), since the absorber includes two rectangular patches The resonant frequency of a specific patch element is proportional to the reciprocal of the side length

of the patch.19So, the electric field orientation determines the effective side length of a rectangular patch

Considering the equations (1) and (2), it can be deduced that for an absorbing element, resonant frequency is inversely proportional to L Simulation results with individual square patch structures verify the dependency of the resonant frequency to the patch size as shown in Fig 8 The linear fit can be used as a powerful design tool for the absorbers The wavelength of waves propagating

in vacuum at 1 THz is approximately 300 µm This value sets a limit for the minimum size of structures in a pixelated array form Fig.8shows that it is feasible to fit various combinations of

different patch geometries in a typical pixel for the operation wavelength The analytical model

of equation 1 suggests that the resonant frequency is independent of the thickness between the electrodes, te ff Although the dependency of the resonant frequency with respect to te ffis weak, we observe that the resonant frequency is altered with the tilt of the freestanding structure as shown in Fig.3(b)

FIG 6 Simulated transmission, reflection and absorption spectra of the absorber.

FIG 7 Distribution of electric field at a) 0.63 THz, b) 1.12 THz, c) 1.47 THz and d)1.87 THz.

III FABRICATION AND EXPERIMENTAL CHARACTERIZATION

We fabricated the designed absorbers using standard microfabrication methods An SEM image for a fabricated structure is shown in Fig 9 The design of this device is shown in Fig.1 The Parylene layer is anchored to the substrate near to the rectangular patches The Parylene layer also

FIG 8 Dependency of the resonant frequency of square patches as a function of side lengths.

FIG 9 SEM image of a fabricated absorber.

includes etch holes that helps removing the sacrificial layer underneath the structure during the releasing step using wet etching We measured the characteristics of the fabricated absorber using

a terahertz time-domain spectroscope (TERA K15, Menlo Systems GmbH, Martinsried, Germany)

We obtained the transmission and reflection spectra of the absorber using reference measurements and calculated the absorption spectrum as shown in Fig.10 We collected the data for the case when

no sample was placed between the emitter and the receiver antennas of the spectrometer as the reference for the transmission measurements Then we placed a thick metal reflector between the antennas that are bent 90◦to collect the reference measurement for the reflectance measurements

FIG 10 Measured transmission, reflection and absorption spectra of the absorber.